The invention relates to a flux-switching linear motor with at least two phases, consisting of a moving rig comprising at least two field coils each surrounding a magnetic armature defining moving magnetic poles and two permanent magnets magnetized in opposite senses, of a guidance stator tube exhibiting magnetic poles disposed along the stator tube, on at least one wall of the stator tube in such a way as to be successively facing the moving magnetic poles during the travel of the moving rig, and of means for switching the sense of the current in the coils.
Most of the known linear motors consist of a moving rig comprising several coils travelling past one or two rows of permanent magnets carried by a rail. Such a motor is described for example in patent application EP 0 161 677. In this motor, the magnets are disposed, with alternating poles, along the whole of one of the inside faces of an amagnetic member, made of aluminum for example, the succession of magnets defining a spacing of periodic structure. In its simplest form, the moving rig consists of two coils fitted with wipers for the DC current supply thereof, switched by two tracks disposed on the opposite inside face of the member. The coils are offset by a quarter of a spacing and supplied in phase quadrature. According to another embodiment, the moving rig consists of three coils offset by a third of a spacing and supplied with currents offset by 120xc2x0.
Another exemplary embodiment is described in patent application GB 2 233 835. In this embodiment, the moving rig, consisting of three coils, overlaps the fixed magnets disposed in the plane of symmetry of a U-profile member.
These structures therefore require magnets over the entire length of the path of the moving rig, although only some of them are active at a given instant. If high-performance permanent magnets are used, the cost of such an installation is considerable and becomes all the more unacceptable the greater the length of the installation. The use of ferrites would enable this cost to be made tolerable, but at the price of mediocre performance. Moreover, simply reducing the length by sawing the linear rail, in the case of a necessary adaptation on-site, becomes a complex operation which requires the prior dismantling of the magnets and careful cleaning of the entire rail after operation. Another drawback appears when the motor is working in an industrial environment: the metal particles encountered in such an environment are attracted by the magnets and interfere with proper operation of the motor.
The motor described in patent EP 0 667 991 remedies these drawbacks. The guidance tube in which the moving rig travels no longer comprises magnets, but is itself made of a ferromagnetic material and is cut in such a way as to exhibit stator poles disposed in two opposite rows, the poles of one row being offset linearly relative to the poles of the other row. Each phase of the moving rig comprises a field coil surrounding an armature consisting of three pole pieces between which are disposed two permanent magnets of opposite polarities, magnetized along the direction of travel of the moving rig. The stator poles and moving poles are arranged in such a way that in a first of two positions of conjunction of the armature, two first moving poles, each belonging to a respective one of the pole pieces, coincide with two stator poles belonging to two different rows of stator poles, while the other two moving poles are offset with respect to the stator poles, whereas in the second of the two positions of conjunction, the other two moving poles coincide in turn with stator poles, the first two moving poles being in turn offset with respect to the stator poles.
Whereas such a motor has the advantage of comprising no fixed magnets, the tubular form of the stator makes it complex to manufacture as far as the precision required to reduce the size of the gap is concerned. Moreover, the mechanical linking of the moving rig to the driven object requires that the stator tube be split on one of its sides, over its entire length, this impeding its magnetic efficiency. Furthermore, the loads applied to the pole pieces of the armature are asymmetric, thereby engendering one or more torsional couples on the moving rig. This drawback makes it essential to considerably rigidify the structure and/or to increase the gap. It is noted, furthermore, that the magnetic leakages existing between the moving rig and the noncut sides of the stator tube degrade the performance of the motor. Finally, the stator tube opposes the bending operations required in order to produce direct actuators, even in the case of a high radius of curvature.
What is needed is a device that remedies the abovementioned drawbacks. In particular what is needed is a way the magnetic efficiency of the stator, to reduce the magnetic leakages and to make it possible to produce curves by bending the guidance stator tube.
The linear motor according to the invention is characterized in that the permanent magnets are disposed outside the coils of the moving rig and that they are magnetized along an axis parallel to the axis, of the coil, and in that the stator magnetic poles consist of pieces made of magnetic material fixed in a guidance tube made of amagnetic material, the dimension of the magnets as measured along their magnetic axis being chosen in such a way as to create narrow gaps for the movement of the magnets past the stator poles.
The stator pole pieces are preferably disposed pairwise, on either side of the axis of the guidance tube, and the dimension of the permanent magnets as measured along their magnetic axis corresponds to the distance separating two opposite stator pole pieces. This disposition has the advantage of eliminating the torsional couple on the moving rig.
According to two embodiments of the invention, the magnets of opposite polarity are disposed symmetrically, either (variant 1) with respect to a plane containing the axis of the coil and a perpendicular to the direction of the stator tube, or (variant 2) with respect to a plane containing the axis of the coil and the axis of the stator tube.
The guidance tube can consist of a U-profile member, as in the case of linear motors with fixed magnets, thereby making it possible to link the moving rig mechanically to a driven object without prejudicing performance. The loads on the moving rig are perfectly symmetrical so that there is no torsional couple. Consequently, modest guidance is sufficient to keep the moving rig in the central position in the guidance tube during its motion. Furthermore, nothing prevents the two or three phases from being articulated either via a ball and socket joint, or simply by utilizing the link between the pairs of alternating magnets in direct contact, in the case of variant 1. A modest gap between two phases, created by a central stud, allows a rotation of one of the phases with respect to the other. In this case, two neighboring field coils are linked by two magnets of opposite polarities.
According to a simplified embodiment, the stator pole pieces are disposed on a single wall of the guidance stator tube and the moving rig is equipped with a magnetic short-circuit plate on the opposite side from the stator pole pieces. The effect of this plate is to create an image of the stator poles.
The neighboring coils may share a common magnet. For a three-phase setup, according to variant 1 or according to the simplified embodiment, the two common magnets are then polarized in opposite senses.
In all cases, the switching of the current in the field coils can be achieved through an AC or DC current supply, as described in the prior art, in particular in patent EP 0 667 991. The switching can be achieved by conducting tracks of suitable shape, as described in patent EP 0 161 677 or be achieved by a switching device mounted on the moving rig, as described in patent EP 0 667 991.
The appended drawing represents, by way of example, a few embodiments of the motor according to the invention.